1. Field of the Invention
A method for verifying the actual location of a seismic sensor array following deployment thereof on the sea floor by a tender vessel. The verification exercise is carried out independently of routine seismic operations.
2. Discussion of Related Art
As is well known, in the art of geophysical surveying and in particular, seismic exploration, an acoustic source launches a wavefield into the earth at a first selected source location along a line of survey. A plurality of seismic receivers or sensors, spaced-apart by a selected spatial interval, are deployed along the survey line for receiving the wavefield after it has undergone reflection from subsurface earth layers. After each wavefield launching, the source or the associated sensors or both are advanced along the survey line by intervals usually equal to a multiple of the sensor spacing. The sensors convert the mechanical reflected seismic signals to electrical signals which are transmitted to a multichannel archival data storage and processing device over wire lines or optical or ethereal signal transmission means of any desired type. As a matter of interest, seismic signals fall into the sub to low audio acoustic band of from about 5 to 100 Hertz (Hz)
In marine seismic surveying, particularly in shallow water on the order of 100 meters or less, one or more arrays of seismic sensors may be mounted in a so-called bottom cable which includes a plurality of data channels. There may be a plurality of individual data channels, one for each sensor, or the seismic signals from many sensors may share a single channel using well-known time-division or frequency division multiplexing methods. The spacing between sensors may typically be about 100 meters. In another type of operation, the sensors may be mounted in anchored buoys and the seismic signals are transmitted to the recording device over one or more ethereal channels.
In shallow-water operation, a cable-carrying boat, guided by a desired type of navigation system such as SYLEDIS or the GPS system, deploys one or more bottom cables at preplotted locations along the intended line of survey. Ten to twenty kilometers of cable may be laid out in one operation as desired. The seismic sensors are built directly into the bottom cable which is designed to be heavy such that it sinks to and remains on the sea floor in contrast to the neutrally buoyant streamer cables that are used in deep-water operations. A recording boat recovers an end of the cable and connects the cable to recording and processing equipment whereupon a shooting boat sails down the line at a desired velocity such as six knots, launching an acoustic pulse at selected spatial intervals in any well-known manner
There may be a considerable period (such as days) between the time that the cable is laid out and the time that the recording boat is ready to gather reflection data. In the meantime, the cable drifts with the sea currents, it may become snagged in fishing trawls and displaced or otherwise moved from its originally intended location. In fact, there is no assurance that the cable, fluttering down through the water as it is being laid out, actually lands at the desired preplotted location on the sea floor. It is therefore necessary to verify the cable and sensor location before commencing data-gathering operations. Verification must be done in real time so that the cable can be repositioned if it has perchance been displaced by more than some pre-ordained spatial tolerance.
U.S. Pat. No. 4,446,538 issued May 1, 1984 to R. G. Zachariadis teaches an acoustic positioning system for locating a marine bottom cable at an exploration site, the cable employs a plurality of hydrophones in spaced-apart positions along the cable. A marine vessel measures water depth to the cable as the vessel passe over the cable and then interrogates the hydrophones with sonar pulses along a slant range as the vessel travels along a parallel horizontally offset path to the cable. The location of the hydrophones is determined from the recordings of the water depth and the slant range. There are two disadvantages to that system, the first of which is that the cable location is not known until after the seismic data have been processed so that if the cable is out of position, a portion of the line would have to be re-surveyed. The other objection is a matter of economy in that the auxiliary boat must make two passes over the cable, one pass to determine water depth and the second pass to generate a set of slant ranges.
Another method for locating a bottom cable is taught by W. P. Neeley in U.S. Pat. No. 4,641,287, issued Feb. 3, 1987. Here is disclosed a method for locating an ocean bottom seismic cable wherein a series of shots from a seismic pulse generator are fired. The distance to one seismic pulse detector is determined for each shot by defining spherical surfaces upon which the detector may be located. The intersection of the spherical surfaces determine the exact location of the detector. Depth detectors may be used to eliminate half the possible locations for each shot. Here again, the range measurements are a by-product of normal seismic data-gathering. The results are not available until after the data have been processed and possibly by that time, the field crew is long gone from the area of survey. In both of the above methods, in relatively shallow water where such bottom cables are used, the range measurements depend on measuring the elapsed time of a first-arriving acoustic pulse that has traveled directly through the water from source to detector and upon knowledge of the water velocity.
Acoustic pulses in the seismic frequency band, in the presence of shallow water, usually follow a direct path to some critical point and thereafter the first-arriving pulse follows a refracted path through the ocean bottom. It is difficult to sort out the direct from the refracted arrivals. Direct arrivals can provide valid range measurements. Refracted arrivals may not do so. Other problems involving ambiguity in acoustic ranging using pulses in the seismic frequency band arise due to reverberation between the water surface and the sea floor and, at long ranges, the probability of wide-angle reflection paths. Thus, acoustic ranging using frequencies selected from the normal seismic frequency band, is unreliable.
A somewhat different location-verification approach is taught by U.S. Pat. No. 5,128,904, issued Jul. 7, 1992 to Ron Chambers and assigned to the assignee of this invention. A method is disclosed for determining the separation between a seismic energy source and a seismic sensor whose location is known imperfectly. After the source emits a wavefield, the first-arriving impulse at the sensor is statistically processed to form a range statistic that is related to the travel time between the source and the sensor. A set of range statistics from a plurality of source positions are filtered and converted to range loci, the intersection of which marks the location of the sensor. Although this method does not suffer from the ambiguous travel-path problems of the two previous references, it is a by-product of data processing so that positioning errors cannot be corrected in the field in real time.
A method that specifically applies to moving neutrally buoyant streamer cables is taught by U.S. Pat. No. 4,532,617 issued Jul. 31, 1985 to D. R. Baeker et al., depends upon acoustic ranging from a slave vessel having known coordinates. That method could, perhaps be adapted for use with an ocean-bottom cable, although it requires cooperation with a master recording vessel.
To reiterate, all of the known methods for verifying the bottom-cable location are a by-product of post-survey data processing. That is, the operator in the field knows not where the seismic transducer array IS but only where is WAS. Furthermore, known methods require use of expensive survey vessels filled with sophisticated seismic equipment operated by senior technicians.
There is a need for a bottom-cable location verification system for use in shallow water that can provide real-time information so that the seismic sensor arrays coupled to a bottom cable can be properly repositioned as needed prior to commencement of the actual seismic survey or the conduct of the survey may be modified to fit the cable location.